Field of the Invention
The present invention relates to a polyarylene sulfide resin composition which can form a molded article having less deformation, excellent surface appearance and gloss and exhibiting excellent mechanical properties for practical use. The resin composition is suitable for the preparation of structural or mechanical parts, exterior trim parts of electrical or electronic appliances, automobiles and other general apparatus, and particularly molded articles requiring high dimensional accuracy.
Recently thermoplastic resins which have good heat resistance, mechanical properties and chemical resistance and excellent flame retardance have been demanded as the material of construction for components of electrical or electronic appliances, automobile devices or chemical instruments. Polyarylene sulfide (PAS) resin represented by polyphenylene sulfide (PPS) is one of the resins satisfying this demand. Due to the relative low cost of PAS resin and the excellent physical properties thereof, the demand for PAS resin has increased.
However, polyarylene sulfide resin has essential disadvantages including poor toughness and brittleness and poor mechanical properties represented by impact resistance. Generally, the addition of a fibrous reinforcement such as glass fiber or carbon fiber or other inorganic fillers has been carried out in order to overcome these disadvantages.
Although such means remarkably improve the strength, stiffness and heat resistance of polyarylene sulfide resin so as to permit the use of the resin as an engineering plastic, a further improvement in the physical properties thereof has been frequently required as the fields of use the resin have widened. In particular, most of the chassis, cases and other precision parts among the parts of various apparatus are required to exhibit not only excellent heat resistance and mechanical properties but also a sufficiently high dimensional accuracy so as not to cause warpage or deformation during molding and service. However, molded articles made from polyarylene sulfide resin compositions containing fibrous reinforcement such as glass fiber suffer from deformation, i.e., "warpage" owing to the enhanced anisotropy of the composition. In order to solve this problem, there have been made many proposals such as the addition of powdery material such as glass bead or flaky reinforcement such as talc, mica or glass flake. Further, attempts have been made to improve both the strength and the deformation resistance by the simultaneous use of a fibrous reinforcement and a flaky material.
Among these reinforcements and fillers, mica is generally inexpensive and is effective not only in reducing the deformation of a PAS molded article but also in improving the electrical characteristics (such as dielectric strength) and heat resistance thereof, thus allowing molded PAS to be useful for electrical components and mechanical parts. However, a molded article made from a mica-filled PAS resin composition is still insufficient in practical strengths, so that a further improvement in the physical properties thereof has been frequently required as the fields of use of molded PAS have widened. Although mica must be added in an amount of at least 5 parts by weight, generally 15 parts by weight or above per 100 parts by weight of a PAS resin in order to reduce the warpage and deformation of a molded article of the resin, a PAS resin composition containing mica in such an amount has other problems. For example, the mechanical properties of a molded article made from the micafilled PAS composition, particularly the strengths of a molded article having a weld are lowered and the surface of an injection molded article made from the composition is rough or poor in gloss so as to lower the sales appeal of the article.
For example, Japanese Patent Laid-Open No. 63355/1982 discloses a polyarylene sulfide resin composition having improved resistance to warpage and deformation, which is characterized by containing a mica, at least 90% by weight of which is composed of particles with a diameter smaller than the opening of a 100-mesh JIS standard screen and which contains magnesium and iron at a weight ratio of Mg to Fe of 2 or above and bonding water in an amount of 2% by weight or below and exhibits a dehydration initiating temperature of 400.degree. C. or above. As described in the above patent document, the mica satisfying these requirements is phlogopite (the so-called "amber" mica) represented by Suzorite mica which contains a relatively large amount of magnesium white. Other micas are unsuitable because they give a composition poor in strengths and toughness. The inventors of the present invention have studied the characteristics of the compositions prepared by adding various micas including amber mica to a PAS resin to find out that a molded article prepared from a composition comprising a PAS resin and amber mica surprisingly exhibits remarkably lowered mechanical strengths, particularly in a weld which is generally present in a molded article, and is unsuitable for practical use with respect to strengths, although the warpage and deformation of the composition upon molding are reduced to some extent. The term "weld" used in this specification refers to the junction of molten resin streams during molding. For example, a weld is always present when molding is conducted by injection through two gates or when an article having a hole or an empty space, such as a ring, is produced by injection molding even through one gate. That is, the presence of a weld cannot be avoided in many molded articles. Accordingly, the lowering in the strengths of a weld is a significant problem in practice. Although the problem can be solved to some extent by controlling the cylinder temperature, injection pressure and mold temperature during molding, the problem is fatal particularly to a mica filled PAS resin.
With the purpose of improving the mica filled PAS resin composition of the prior art in mechanical strengths, particularly the weld strength of a molded article, and surface appearance while keeping the excellent deformation resistance, heat resistance and electrical properties of the composition and thereby providing a practically useful composition having well balanced characteristics, the inventors of the present invention have made comparative studies on various micas to find out that a specific mica which has hardly been used as a filler for plastics is effective for the above purpose. The present invention has been accomplished on the basis of this finding.